A man named Shouichi Tsugami has lost his memories. He doesn't know who he is, where he came from, or how he came upon his peculiar circumstances. Tsugami, seemingly for no reason, transforms into a powerful superhuman, Kamen Rider Agito, whenever in the presence of the beings referred by the police as the "Unknown", a race of powerful monsters that have been causing murders around Tokyo, targeting certain people as their prey.